Download Pigments in Forensic Geology

Minerals
Modified from a PowerPoint presentation prepared by J.
Crelling, Southern Illinois University
Minerals
• Building blocks of rocks, soil ,dirt, and mud
• Minerals are everywhere
• Rocks are aggregates of one or more minerals
Mineral Definition
1. Naturally Occurring
2. Inorganic
3. Crystalline – has a definite internal
structure, i.e., atoms in the mineral are
arranged in a regular way
4. Chemical composition fixed or varies within
certain limits
Minerals
• Naturally Occurring - minerals must be
formed naturally - glass, concrete,
synthetic diamonds, rubies and
emeralds don’t count
• Inorganic - minerals are not formed by
anything that was ever alive.
Therefore, materials such as:
Ivory, Amber, Coal, Pearls
are not minerals!
Minerals
• Crystalline - the atoms in minerals have
an orderly atomic arrangement giving
them a definite structure that controls
their properties.
Structure of minerals
• Minerals consist of an orderly array of
atoms chemically bonded to form a
particular crystalline structure
• Internal atomic arrangement in ionic
compounds is determined by ionic size
Geometric packing
of various ions
Figure 3.8
Structure of Minerals
Crystal Lattice: the three
dimensional molecular
structure of a mineral.
(Shape of the “unit cell.”)
• Various ions make up the
mineral.
• Geometry + chemistry!
Structure of Minerals
Halite (rock salt)
Structure of minerals
Polymorphs
• Minerals with the same composition
but different crystalline structures
• Examples include diamond and
graphite
• Phase change – one polymorph
changing into another
Structure of Minerals
Diamond
Graphite
Polymorphs
of Carbon
Physical properties
of minerals
Crystal Form
• External expression of a mineral’s
internal structure
• Often interrupted due to competition for
space and rapid loss of heat
Crystals
Crystals are the smallest “bits” of
minerals and reflect the geometry
of the mineral molecules
A garnet crystal
Cubic crystals of pyrite
Figure 3.11 A
OLIVINE
TOURMALINE
QUARTZ
Physical properties
of minerals
Color
• Generally unreliable for mineral
identification
• Often highly variable due to slight
changes in mineral chemistry
• Exotic colorations of certain minerals
produce gemstones
• Some minerals are used as pigments
Quartz (SiO2) exhibits a variety
of colors
Physical properties
of minerals
Streak
Color of a mineral in its powdered form
Streak is obtained on an
unglazed porcelain plate
Figure 3.12
Physical properties
of minerals
Luster
• Appearance of a mineral in reflected light
• Two basic categories
• Metallic
• Nonmetallic
• Other descriptive terms include vitreous,
silky, or earthy
Galena (PbS) displays
metallic luster
Physical properties
of minerals
Hardness
• The hardness of a mineral is its resistance to
scratching.
• The standard scale for measuring hardness
is Moh’s Hardness scale.
Mohs scale
of
hardness
Physical properties
of minerals
Cleavage
• Tendency to break along planes of weak
bonding
• Produces flat, shiny surfaces
• Described by resulting geometric shapes
• Number of planes
• Angles between adjacent planes
Fluorite, halite, and calcite all
exhibit perfect cleavage
Cleavage
– Planes of weakness caused by common
crystal faces being aligned
Fluorite Crystals
Fluorite Cleavage
Fragments
Physical properties
of minerals
Fracture
• Absence of cleavage when a mineral is
broken
Specific Gravity
• Weight of a mineral / weight of an equal
volume of water
• Average value = 2.7
Physical properties
of minerals
Other properties
•
•
•
•
•
•
•
Magnetism
Reaction to hydrochloric acid
Malleability
Double refraction
Taste
Smell
Elasticity